1. Field of the Invention
The present invention relates to a semiconductor storage device, and in particular, to a non-volatile semiconductor storage device allowing multiple bits to be stored in one memory cell.
2. Description of the Related Art
NAND-type flash memory is known as one of non-volatile semiconductor storage devices. The NAND-type flash memory has a memory cell array including a plurality of NAND cell units. Each NAND cell unit includes a plurality of memory cells connected in series and two selection transistors connected to both ends thereof.
Each memory cell stores, in an erase state, data “1” having a negative threshold voltage. In a data write operation, electrons are injected into a floating gate to write data “0” having a positive threshold voltage. The NAND-type flash memory may change the threshold voltage only from a lower value to a higher value in a data write operation, and may change the threshold voltage in the reverse direction (from a higher value to a lower value) only by an erase operation on a block basis.
To increase memory capacity, current technology has developed so-called multi-value NAND-type flash memory to store two or more bits of information in one memory cell. For example, when 3 bits are stored in one memory cell, one memory cell involves 23=8 different threshold voltage distributions. If 8-value information is stored in one memory cell, the interval between eight different threshold voltage distributions will be reduced. Thus, incorrect data read will occur in a data-read operation, which may lead to lower data reliability.
This requires an error check and correct (ECC) operation to be performed for correcting any incorrectly read data. The error check and correct operation may control the number of write pulses to be applied by performing a verify operation when writing data to a memory cell (see Japanese Patent Laid-Open No. 2006-12367), or correct data that has been read from a memory cell when reading data.
For a semiconductor storage device where an error check and correct operation is performed in reading data, improved data reliability may be achieved by reading data to be read as well as the threshold voltage information of the relevant memory cell, and adding that information to the read data. As used herein, the term “threshold voltage information” refers to information that indicates where the threshold voltage of a memory cell is positioned in the threshold voltage distribution to which certain data is allocated. Given that operations are separately performed for reading data and threshold voltage information, the read data that has been read by a sense amplifier and retained in a data register is transferred to the outside before the threshold voltage information is read. Since the NAND-type flash memory outputs a large amount of data, there will be some period of time during which threshold voltage information is not read at the time of transferring read data, which could increase the total operating time for reading data and threshold voltage information.